1. Field of the Invention
The invention relates to a tube for heat exchangers and a method for manufacturing the tube, more particularly, the tube being of a flat or depressed shape adapted to compose the multiflow heat exchangers which are used as condensers in car cooler systems.
2. Description of Prior Art
The condensers in car cooling systems have generally been heat exchangers of the so-called serpentine-tube type. The principal parts of such prior art heat exchangers comprise a "harmonica" tube and fins combined therewith, this tube being a flat extruded tube having internal and longitudinal openings and being bent zigzag several times to thereby form portions parallel with one another, with each fin being disposed between those portions.
Another kind of prior art heat exchanger is called a "multiflow" type, and has recently been proposed and employed to reduce the flow resistance of coolant, to improve the heat transfer efficiency, to reduce the weight and the volume of the condenser. The multiflow type heat exchanger comprises, for example as shown in FIG. 13, a pair of right and left headers 31 and 32 made of a metal pipe. A plurality of flat tubes 33 are connected at their ends to the headers in fluid communication therewith. Fins 34 are each interposed between two adjacent tubes 33 and 33. Partitioning members 35 are each secured inside the headers 31 and 32 at suitable positions intermediate of their ends so that internal spaces of the headers are divided into compartments. Thus, a coolant passage of a zigzag pattern is formed to start from a coolant inlet 36 at an upper end of one header 31 and then to terminate at a coolant outlet 37 at a lower end of the other header 32 (as disclosed, for example, in the U.S. Pat. No. 4,825,941).
The abovementioned tubes 33 in the multiflow type heat exchangers have in general been certain flat or depressed aluminum tubes which are produced by the extrusion forming method and comprise longitudinally extending flow paths, because the tubes must withstand the high pressure of the compressed gaseous coolant employed in the heat exchanger. As shown in FIG. 14, each of those tubes has a peripheral wall 33a which is in the shape of ellipse in cross section. Each tube has also one or more longitudinal partitions 33b to divide the internal space into separate coolant paths 33c.
However, in all cases wherein the extruded tubes 33 are employed, their height "H" which is restricted by the manufacturing process, preventing the heat transfer efficiency from being raised above a certain upper limit. As will be understood, higher efficiency of heat transfer within a heat exchanger may be achieved effectively by minimizing the flow resistance of air which flows through the core of a given contour dimension, and at the same time, by increasing the core's overall surface in contact with the air flow. In other words, the extruded tubes 33 of a height "H" which has not been lowered to a sufficient degree have caused an increase in the air flow resistance and placed restrictions on the number of tubes installed within each core of a given contour dimension, thus failing to increase the core's surface area in contact with the air flow.
Seam-welded pipes have been proposed for use as the tubes in order to eliminate such a drawback (for example, see the Japanese Patent Publication 62-207572). The wall of seam-welded tubes can be reduced to a thickness of about 0.4 to 0.5 mm, remarkably decreasing the tube's height to about 1.5 to 1.7 mm.
Such an extremely thin wall per se of the seam-welded tubes cannot withstand the high pressure gaseous coolant which is supplied from a compressor to the tubes of the condensers. To resolve this problem, the prior art as disclosed on said Patent Publication 62-207572 makes use of an inner fin member inserted into each flat seam-welded tube. Those inner fin members which are previously corrugated in transverse direction before insertion are each soldered to the inner surface of tube so as to function also as a reinforcing member which enhances the tube's resistance to pressure.
The prior art tubes which are seam-welded and reinforced are not necessarily easy to manufacture. Particularly, it is considerably difficult to insert the inner fin member the entire length of each tube which is extremely thin, whereby productivity is lowered raising the manufacturing cost.